A transistor formed from a wide band gap semiconductor represented by GaN, SiC, or the like has excellent characteristics of having high-speed switching, a low ON-state resistance value, and low capacitance compared to a transistor formed from a silicon semiconductor. Therefore, it is expected that it will be applied to a control apparatus or a power converter such as an AC/DC converter and an inverter.
Some transistors formed from a wide band gap semiconductor exhibit a normally-ON property in which a threshold voltage becomes negative. However, since it is often a burden in circuit design to provide a negative gate voyage, a configuration illustrated in FIG. 8 is preferably adopted, in which a multiple-unit semiconductor device 900 is formed by cascode-connection of a GaN transistor 901 that is a normally-ON type transistor and a MOSFET 902 that is a normally-OFF type transistor, and operates as one transistor at a positive gate voltage.
The multiple-unit semiconductor device 900 includes a drain terminal 911 to which a power supply voltage VDD is applied, a source terminal 912 connected to the ground, and a gate terminal 913. The drain terminal 911 is connected to a drain of the GaN transistor 901, and the source terminal 912 is connected to a source of the MOSFET 902, and a source of the GaN transistor 901 and a drain of the MOSFET 902 are commonly connected to each other. A gate of the GaN transistor 901 is connected to the source terminal 912 via a resistance element 903 and a gate of the MOSFET 902 is connected to the gate terminal 913. A substrate terminal of the GaN transistor 901 is connected to the ground.
In this configuration, when a gate voltage of the MOSFET 902 equal to or higher than a positive threshold voltage is applied to the gate terminal 913, the MOSFET 902 is in an ON state, and thus, the state between the terminals 911 and 912 becomes conductive. In a case where a gate voltage to the gate terminal 913 is lower than the threshold voltage, the MOSFET 902 is in an OFF state and a voltage lower than the threshold voltage of the GaN transistor 901 is generated between the gate and source of the GaN transistor 901, and thus, the GaN transistor 901 is in an OFF state. Therefore, the state between the terminals 911 and 912 becomes non-conductive. That is, the multiple-unit semiconductor device 900 realizes an operation as one transistor.
In many cases of forming a product, two chips (a GaN transistor 901 chip and a MOSFET 902 chip) are accommodated in one package, and thus, a user can use the multiple-unit semiconductor device 900 as if the user treats an ordinary transistor.
On the other hand, in order to protect the internal circuit from a surge due to static electricity generated by a human body or a machine, that is, an electro-static discharge (ESD) surge, an ESD protection circuit is usually connected to or embedded in a semiconductor device or an integrated circuit. The ESD protection circuit operates only when the ESD surge is applied, and makes an ESD surge current immediately flow to the ground side. In this way, the damage to the internal circuit by the ESD surge is reduced.
In the PTLs 1 and 2 below, configurations for protecting an internal circuit from an ESD surge are disclosed.